If Cd[Zn]Te with not fully blocking (cathode) contacts is used for single photon counting detectors, the photoconductive gain causes a major problem especially for DC coupled readout electronics: Since it is a slowly changing current on top of the photocurrent, it causes a baseline shift at the output of the analog readout channel towards the set energy thresholds so that, without any correction means, the energies of the photons is wrongly registered. If the persistent current changes very slowly, and pile-up is limited, known approaches like conventional baseline restoration (BLR) can be applied. It is well-known that the conventional BLR approach of sensing the baseline (BL) at the output of the shaper (SHA) will result in significantly wrong baseline estimates, if pulses at the output of the SHA pile-up frequently: In this case, the BL is no longer reached so the BL is wrongly estimated.
An additional effect may be caused by induced pulses from neighbor pixels, which have a bipolar waveform. Depending on the phase of an induced pulse relative to the signals of real pulses, the induced pulse can contribute to pile-up or reduce the pile-up visible on the SHA output signal.
Further, even at low x-ray fluxes but long irradiation times the sensor gets polarized. This is the case during the energy calibration of the detector in which the correct threshold positions should be determined. Polarization distorts the calibration.
In addition to the above, baseline restorer circuits are typically implemented by using a peak detector that senses the baseline, a low pass filter (e.g. an integrator) to restrict to the compensation of low frequency BL shifts and (a) transconductor element(s) in charge of injecting or sinking a compensating current at the input node (or shaper input depending upon implementation). The peak detector within the BLR circuit is however very sensitive to excursions of the shaper output level in the opposite direction of the expected background current, particularly above the BL (in an implementation where the shaper output signal is a pulse below this BL). That is, a signal above the BL level will be sensed by the peak detector as if it were the new BL level, causing a correction equal to the full signal excursion above the BL level in the worst case. Such excursions above the BL level can primarily be caused by two non-ideal artifacts; namely induced pulses (which have a bi-polar shape around the BL level) from neighboring pixels and shaper overshoot (a small half wave above the BL level).
US2010172467A1 relates to an apparatus for generating countable pulses from impinging X-ray in an imaging device, in particular in a computer tomograph. The apparatus comprises a pre-amplifying element adapted to convert a charge pulse generated by an impinging photon into an electrical signal and a shaping element having a feedback loop and adapted to convert the electrical signal into an electrical pulse. A delay circuit is connected to the feedback loop such that a time during which the feedback loop collects charges of the electrical signal is extended in order to improve an amplitude of the electrical pulse at an output of the shaping element.
US2004027183A1 discloses a continuous-time baseline restoration (BLR) circuit providing built-in pulse tail-cancellation, or BLR tail-cancel circuit, in constant fraction discriminator (CFD) arming and timing circuits. The BLR tail cancel circuit is applied at the output of constant fraction timing shaping filters and arming circuits to permit monolithic integrated circuit implementation of CFD circuits operating at high input signal count rates. The BLR tail-cancel circuit provides correction of dc offset and count-rate dependent baseline errors along with simultaneous tail-cancellation. Correction of dc offsets due to electronic device mismatches and count-rate dependent baseline errors is required for accurate time pickoff from the input signals. The reduction of pulse width, or pulse tail-cancellation is required to shorten the duration of high count rate signals to prevent the severe distortion caused by the occurrence a new signal superimposed on the tails of previous signals, a condition known as pulse pileup. Without pulse tail-cancellation, there are substantial errors in time pickoff due to the pulse pileup.
WO2013057645A2 discloses an imaging system includes a detector array with direct conversion detector pixels that detect radiation traversing an examination region of the imaging system and generate a signal indicative of the detected radiation, a pulse shaper configured to alternatively process the signal indicative of detected radiation generated by the detector array or a set of test pulses having different and known heights that correspond to different and known energy levels and to generate output pulses having heights indicative of the energy of the processed detected radiation or set of test pulses, and a thresholds adjuster; configured to analyze the heights of the output pulses corresponding to the set of test pulses in connection with the heights of set of test pulses and a set of predetermined fixed energy thresholds and generate a threshold adjustment signal indicative of a baseline based on a result of the analysis.
The article “Counting and Integrating Readout for Direct Gonversion X-ray Imaging: Concept, Realization and First Prototype Measurements” by E. Kraft et al., IEEE Transactions on Nuclear Science, volume 54, pages 383 to 390 (2007) discloses a signal processing concept for X-ray imaging with directly converting pixelated semiconductor sensors. The approach combines charge integration and photon counting in every single pixel. Simultaneous operation of both signal processing chains extends the dynamic range beyond the limits of the individual schemes and allows determination of the mean photon energy. Medical applications such as X-ray computed tomography can benefit from this additional spectral information through improved contrast and the ability to determine the hardening of the tube spectrum due to attenuation by the scanned object. A prototype chip in 0.35-micrometer technology has been successfully tested. The pixel electronics are designed using a low-swing differential current mode logic. Key element is a configurable feedback circuit for the charge sensitive amplifier which provides continuous reset, leakage current compensation and replicates the input signal for the integrator. The article discusses measurement results of the prototype structures and gives details on the circuit design.
The article “ChromAIX: a high-rate energyresolving photon-counting ASIC for spectral computed tomography” by R. Steadman et al., Proceedings of SPIE, pages 762220-762220-8 (2010) discloses a study of the feasibility of Spectral CT. An energy-resolving proprietary photon counting ASIC (ChromAIX) has been designed to provide high count-rate capabilities while offering energy discrimination. The ChromAIX ASIC consists of an arrangement of 4 by 16 pixels with an isotropic pitch of 300 micrometers. Each pixel contains a number of independent energy discriminators with their corresponding 12-bit counters with continuous read-out capability. Observed Poissonian count-rates exceeding 10 Mcps (corresponding to approximately 27 Mcps incident mean Poisson rate) have been experimentally validated through electrical characterization. The measured noise of 2.6 mVRMS (4 keV FWHM) adheres to specifications. The ChromAIX ASIC has been specifically designed to support direct-converting materials CdZnTe and CdTe.
A reduction of artifacts due to inherent errors with direct conversion detectors in spectral computed tomography (CT) is desirable.